In electrostatic image transfer a toner or ink image defined on an image support surface is transferred electrostatically to a substrate. It may be fixed on that substrate, or alternatively transferred to a further substrate which is the intended image carrier.
In conventional electrostatic copiers, a developed image is transferred to loosely held paper by electrostatic and adhesion forces. The loosely held paper is brought into contact with the image as it is simultaneously charged to a high voltage. This technique, which is adequate for medium quality photocopying is unsuitable for more exacting processes, such as color proofing, for two principle reasons: loose mounting of the paper makes registration of one image with another difficult, and mottle results from topographical variations in the loosely held paper surface. Such variations result in variation in the pressure of the paper on a wet image, which result in variations in dot size. The variations in dot size translate into low frequency grey-level variations, here referred to as "mottle".
The use of tensioned substrates in electrostatic image transfer is known in the literature. U.S. Pat. No. 4,015,027 describes an electrophotographic toner transfer and fusing method which employs a tensioned transfer belt for transferring a toner image from a photoconductive drum or a photoconductive insulating sheet onto paper.
In the embodiment of U.S. Pat. No. 4,015,027, the transfer belt is tensioned only along the direction of its travel. Such tensioning tends to produce surface irregularities in the belt along directions transverse to the direction of travel. As a result the uniformity of transfer is adversely affected, lowering the general level of image quality produced by the apparatus due to mottle and rendering the transfer technique incompatible with registration requirements as for multi-color operations.